MCQ
$\cos \frac{\pi }{7}\cos \frac{{2\pi }}{7}\cos \frac{{4\pi }}{7} = $
- A$0$
- B$\frac{1}{2}$
- C$\frac{1}{4}$
- ✓$ - \frac{1}{8}$
$= \left[ {\frac{{\sin \left( {{2^3}.\frac{\pi }{7}} \right)}}{{{2^3}\sin \left( {\frac{\pi }{7}} \right)}}} \right] $
$= \frac{{\sin \frac{{8\pi }}{7}}}{{8\sin \frac{\pi }{7}}}$
$= - \frac{1}{8}$.
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$FACT$ : If $a$ and $b$ are rational numbers and $a+b \sqrt{5}=0$, then $a=0=b$.
($1$) $a_{12}=$
$[A]$ $a_{11}-a_{10}$ $[B]$ $a_{11}+a_{10}$ $[C]$ $2 a_{11}+a_{10}$ $[D]$ $a_{11}+2 a_{10}$
($2$) If $a_4=28$, then $p+2 q=$
$[A] 21$ $[B] 14$ $[C] 7$ $[D] 12$
answer the quetion ($1$) and ($2$)